My son has a 1974 Duster, automatic with, I believe, the 225 inch version (could be a 198). The car was bought with 60K orig miles from the estate of the original 90 year old owner. It runs great but does not get the mileage I would expect. We rebuilt the original Calif. carburetor, replaced fuel pump, did valve job and tune up and it really purrs! But driving it to and from college, all freeway, a 75 mile round trip, he averages 16 MPG ! I have had V8's that did better and my Old '70 Duster with a 198 got over 25-26 MPG Highway with three speed. Any ideas? I have not messed with the timing because it seems just right. The tires are full. The car has NO options and doesn't even have carpets (its rubber mats!). No leaks that we can tell.
What are your ideas? The car has the original 1974 smog stuff but I have plugged the vacuum hose to the air cleaner so its always open to fresh air. Could it be automatic choke? No smog pump on this model!
Help!
KJSheridan and Son

Welcome to the board KJ. Where are you guys at in CA?

What kinds of smog stuff do you have on this car? Does the vacuum line from the distributor to the carb have any type of smog deal connected along the way? If it does then disconnect it and let that line go straight from the distributor to the carb without any obstructions.

How fast was your son driving? If he was doing 85 the whole way then 16mpg is not too far out of line. This motor is old and even though it has relatively low miles, I'd say that you'll probably only be able to max out at 20mpg unless you start throwing some money at it.

Check your choke and make sure it pulls all the way off after a couple minutes. Check your timing. The sticker probably says TDC but you want it between 5*-10* BTDC. How did you do your valve job? Did you have the motor running as the instructions state on here? If so did you go .010 intake and .020 exhaust? Also how are the spark plugs? And I can't remember, in 74 did they go to electronic ignitions or does it still have points?

If the dist. vacuum advance ruptures, it'll usually still run pretty well, plugs will look good, but you'll loose a lot of mileage. It's easy to test & replace.

The Duster has the standard Electronic Ignition. The distributor hose now goes directly to the carb. My son says he is going a legal speed. We are in Los Angeles so 65 to 70 is the max. He occassionally gets some traffic but not that much. The valve job was done by a prof. mechanic and the engine really smoothed out and quieted down after the valve job. It tapped a lot before and I thought it was just low on oil. I didn't realize it had solid lifters! The performance improved a bit too. Thanks for your help and ideas. I will check the Vacuum advance and the timing.
KJSheridan

Also check for timing chain stretch and vibration dampener slip.

You are right, you should get between 18-20MPG with this car.

1. Check for voltage drop from the alt B+ and the high side of the ballast resister. Voltage drops there will cost you mileage.

2. Run a can of good top end cleaner through the carb, and get the carbon cleaned out of the head and valves. I used to use a variety sold by Amsoil, called Power Foam. It worked great. The word there is right after running it through the engine, don't shut is off, but take it out and drive it hard. These are two basic, simple things you can do to try and get the car back to functioning like it should.

Sam

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I called the mechanic who did the valve adjustment and asked him about the vacuum advance diaphragm. It turns out it was bad when he did the new carburetor and he replaced it then. He will check it again for me this next week to make sure it is still OK. As to voltage drop to the Ballast Resister, what should the voltage be? 13.5 or so? How would the vibraton damber slip? Isn't it solidly bolted in place? I am thinking you are talking about the harmonic balancer, right?
KJSheridan

The outer ring of the harmonic balancer(the part with timing marks on it) is held on by a thin rubber ring between the hub and outer ring-they do slip some times (rotate) on the hub and throws you timing marks off. More of a age thing and not milage.

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50 chrysler,54 Plymouth, 64 Valiant conv 4 speed, 66 Valiant V8 wagon, 70 Challenger R/T 440+6 conv 4 speed,80 Colt, 98 Neon ACR,84 Honda V45 Magna
Taking care of 57 300C conv,48 T&C conv. Missing my 67 GTX and 36 Ply coupe

My 74 Swinger with a 225 and 904 automatic got about the same mileage when I got it.

On the voltage drop to the coil I was running as low as 4 volts at an idle. Mileage was around 18 mpg. Since then I have gotten it up to 23.5 to 24 mpg for local driving and 25 plus on longer drives.

After replacing the engine wiring harness, connectors and one wire at a time I got it up to 10 volts at the coil (opposite side of the ballast resistor). The pictures show I upgraded from the dual ballast to a single .85 ohm resistor. Several wires had gotten hot around the large alternator wire and burned off the enamel and were shorting also causing the oil light to flicker. The previous owner had removed the oil light bulb so I didn't know. You couldn't see the shorting wires until I started unraveling the tape around the harness.

Currently I replaced the ballast resistor and bought a 3 ohm coil. Yes you can do that by putting a jumper wire in place of the ballast resistor. Now I am getting a full 12+ volts to the coil.

Another thing to consider is cleaning up the distributor since you have to replace the vacuum canister. Mine was not advancing very much do to the hardened grease between the advance plate. The counter weight posts were also rusty which caused binding and would not allow the weights to fully extend. So I was not getting much over 10 degrees mechanical advance. Cleaned up I am getting 20 degrees. Look under Engine FAQ for instructions on how to dial in the vacuum canister.

With the initial set at 12 degrees, not zero per the book, I know have 32 degrees advance plus another 22 degrees from the vacuum advance.

Click on the red link below my name to view the changes. Double click on the picture once it comes up for full screen slide show mode.

You may want to keep the 1945 carb and rebuild it. I did, but ended up changing over to the 1920 Holley EconoMaster version after a few years of driving it. I can get a little better mileage from the 1920.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

He might just be going fast enough to dip into the power circuit of the carb making it run rich............

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Whatever the voltage reading is on the B+ at the alt at idle, it should that on the high side of the ballast resister. Put your plus probe on the big red wirie nut on the back of the alt, and your negative probe anywhere to ground. Then run the same test all along the path to the starter motor, and the blue and crown wires that exit the bulkhead co0nnectors headed to the ignition/ballast. If the voltage is not the same, you have high resistance in your wires, the spade connectors, the bulkhead connectors, or the terminals at the ammeter. Most likely it is all of those. A drop of 1/2 volts is common, but still considered a problem by me. I drop of 1.5 volts happens often, and is really going to be causing you problems.

The two things that helped my car the most when it was still stock was improving the wiring, and running a top end cleaner through the carb.

You can take your VOM and check the voltage all along the charging, and ignition circuit , and find out precisely where the voltage drops are. It is a check that costs you nothing to do, and is very illuminating.

Sam

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Ask the mechanic if he did your valve adjustment with the engine running or not. If he did it while the motor was off, then your valves are most likely too tight. I can make my engine purr and get 17mpg or I can make it purr at 23mpg. Depends on my timing, valve lash and carb tuning. Just because it's running smoothly doesn't always mean it's running at it's best.

This is a good possibilty, a stock 1974 Holley 1945 with a #58 is lean enough that at highway passing speeds the vacc. gauge would dip below 6" if you are over 1/2 throttle (and that was on my '74 with a 3 spd stick shift and a good rear ratio...)

-D.Idiot

Ceej,

Good point!

I never new what was going on until I installed a vacuum gauge. It is the best investment I ever made. It has forced me to keep the engine tuned up! and the valve lash loose (.012 and .022 set while running). I drive by the gauge religiously and as a result get some great mileage.

I agree a #58 jet is too small for the 1945. It seems to get better mileage with a #61 jet.

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Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG

Also a sick engine will do that. Like a cylinder one or two firing weakly (due to less compression). Only make idle vibration change a little but more noticeable if weather is much colder like 0C.

Cheers, Wizard